The mammalian brain is vigilant in the control of breathing, regulating blood O2 and CO2 over an order of magnitude range in metabolism, wide ranges of posture and body movements, and compromises in muscle or cardiopulmonary function from birth till death without lapses beyond a few minutes. The long-term objectives of this grant are to understand the mechanisms by which the central nervous system generates respiratory rhythm and modulates respiratory pattern to appropriately regulate blood gases and pH. In this grant, we propose to test, in behaving awake/sleeping adult rats hypotheses related to the sites and mechanisms for generation of breathing rhythm. In particular, we will test the hypothesis that the preBotzinger Complex, a compact brainstem nucleus, contains the critical circuits generating the normal rhythm of breathing. The experiments will determine the effects of destroying targeted populations of preBotzinger Complex neurons in adult rats and measuring the changes in breathing pattern and related physiological variables that result. Our published results indicate that destroying a small population of less than 1000 neurokinin-1 receptor expressing preBotzinger Complex neurons will result in pathological breathing in awake rats, as well as severely perturb their ability to respond to hypoxia; preliminary data indicate marked disturbances following destruction of opioid receptor expressing neurons. Understanding the neural mechanisms producing respiratory rhythm and underlying its modulation appropriate for eupnea, i.e., the breathing pattern necessary to keep the arterial partial pressure of O2 and CO2 and pH at physiological levels when at rest, is important in understanding human disease. Failure of the brain to maintain eupnea in humans suffering from a variety of disorders, including sleep apnea, apnea of prematurity, congenital central hypoventilation, central alveolar hypoventilation, and perhaps sudden infant death syndrome, leads to serious adverse health consequences, even death. If these pathologies are to be understood, the site(s) and mechanism(s) of respiratory rhythmogenesis must be revealed.